Java™ How To Program (Early Objects), Tenth Edition

Method displayState Is Also synchronized

Notice that method displayState is a synchronized method. This is important because it, too, reads the SynchronizedBuffer’s shared mutable data. Though only one thread at a time may acquire a given object’s lock, one thread may acquire the same object’s lock multiple times—this is known as a reentrant lock and enables one synchronized method to invoke another on the same object.